Split-shell fractionation columns and associated processes for separating aromatic hydrocarbons

ABSTRACT

Split-shell fractionation columns and associated processes for separating aromatic hydrocarbons are provided by using, a split-shell fractionation column includes a housing shell having a first height and a partition having a second height and disposed within the housing shell. The partition includes first and second vertically oriented baffles separated by a gap region, a seal plate connecting top ends of the baffles, a first input port formed to extend through the partition for the introduction of a gas into the gap region, and a first output port formed to extend outwardly from a bottom of the gap region and through the housing shell. The partition defines a first distillation zone and a second distillation zone within the housing shell.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.13/715,774 filed Dec. 14, 2012, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The technical field relates generally to chemical separation processesand associated apparatus. More particularly, the disclosure relates toprocesses for the separation of an aromatic hydrocarbon isomer, forexample a xylene isomer, from a feed stream containing a mix of aromaticand non-aromatic hydrocarbons using a split-shell fractionation column.

BACKGROUND

Aromatic hydrocarbons find a plurality of uses in various chemicalsynthesis industries. In one non-limiting example, para-xylene is animportant intermediate aromatic that finds wide and varied applicationin chemical syntheses. Upon oxidation, para-xylene yields terephthalicacid. Polyester fabrics and resins are produced from a polymer ofethylene glycol and terephthalic acid. These polyester materials areused extensively in a number of industries and are used to manufacturesuch items as, for example, clothing, beverage containers, electroniccomponents, and insulating materials.

In prior art processes, C₉ aromatic hydrocarbons are separated from C₈aromatic hydrocarbons, for example xylene isomers, by fractionaldistillation. This requires heating of the admixture to vaporize the C₈and lighter aromatic hydrocarbons. A large portion of the isomerizationstream must be vaporized to accomplish the C₉ separation because thestream is generally composed primarily of C₈ and lighter aromatichydrocarbons. After the C₉ aromatic removal, the C₈-containing stream isthen recycled into an adsorptive separation unit. Multiple, largefractionation columns are often required to accomplish these processsteps. As such, this separation process requires a substantial amount ofenergy and associated capital costs.

The production of aromatic hydrocarbon isomers, including for examplepara-xylene, is practiced commercially in large-scale facilities and ishighly competitive. A never-ending drive exists to decrease the energycosts and capital costs yet increase the effectiveness associated withthe conversion of a feedstock through one or more of isomerization,transalkylation, and disproportionation to produce select isomers andseparate the select isomers from the resultant mixture of C₈ aromaticisomers.

Accordingly, it is desirable to provide processes for the production ofparticular aromatic isomers, including the separation of such isomersfrom an admixture of C₈ and C₉ aromatic isomers, that lowers operationalexpenses, particularly energy consumption. In addition it is desirableto provide processes for the production of particular aromatic isomersthat lowers capital expenditures, in the form of processing equipmentand the size of such processing equipment. Further, it is desirable toprovide split-shell fractionation columns for use in such processes.These and other desirable features and characteristics will becomeapparent from the subsequent detailed description and the appendedclaims, taken in conjunction with the accompanying drawings and theforegoing technical field and background.

BRIEF SUMMARY

Split-shell fractionation columns and associated processes forseparating aromatic hydrocarbons are provided herein. In one exemplaryembodiment of the present disclosure, a split-shell fractionation columnincludes a housing shell having a first height and a partition having asecond height and disposed within the housing shell. The partitionincludes first and second vertically oriented baffles separated by a gapregion, a seal plate connecting top ends of the baffles, a first inputport formed to extend through the partition for the introduction of agas into the gap region, and a first output port formed to extendoutwardly from a bottom of the gap region and through the housing shell.The partition defines a first distillation zone and a seconddistillation zone within the housing shell.

In another exemplary embodiment of the present disclosure, a process forseparating aromatic hydrocarbons includes the steps of introducing afirst stream including a plurality of aromatic hydrocarbons into a firstdistillation zone of a split-shell fractionation column and introducinga second stream including a plurality of aromatic hydrocarbons into asecond distillation zone of the split-shell fractionation column. Thefirst and second distillation zones are defined by a partition withinthe split-shell fractionation column. The partition has a gap regionlocated therein. The process further includes the step of separating thefirst stream into a first overhead product and a first bottom product.The first bottom product includes a first liquid that collects at abottom portion of the first distillation zone. The process furtherincludes the step of separating the second stream into a second overheadproduct and a second bottom product. The second bottom product includesa second liquid that collects at a bottom portion of the seconddistillation zone. Still further, the process includes the step ofdraining the first liquid, the second liquid, or a combination thereofin the gap region through a first outlet port.

BRIEF DESCRIPTION OF THE DRAWINGS

The split-shell fractionation column and its associated processes willhereinafter be described in conjunction with the following drawingfigures, wherein like numerals denote like elements, and wherein:

FIG. 1 is a schematic view of an exemplary embodiment of a split-shellfractionation column in accordance with the present disclosure; and

FIG. 2 is a diagram of an embodiment of a process for separatingaromatic hydrocarbons employing a split-shell fractionation column as inFIG. 1.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the various embodiments or the application anduses thereof. Furthermore, there is no intention to be bound by anytheory presented in the preceding background or the following detaileddescription.

Para-xylene, or any other select aromatic hydrocarbon isomer as may bedesired, is typically recovered from a mixed aromatic hydrocarbonfraction derived from various sources such as catalytic reforming ofpetroleum by adsorptive separation, liquid-liquid extraction, and/orfractional distillation. The select aromatic isomer is then separatedfrom that fraction, which typically contains all three xylene isomers,namely ortho-xylene, meta-xylene, and para-xylene. The para-xylene, orother desired isomer, is separated from the fraction to isolate thedesired isomer.

FIG. 1 illustrates an exemplary embodiment of a split-shellfractionation column 210. Split-shell fractionation column 210 includeshousing shell 102, an upper portion 120, and a lower portion 122. Thehousing shell 102 has a height 155. Upper portion 120 includes aplurality of column trays 103. Column trays 103 may be implemented asconventional fractionation column trays as are well-known in the art.Lower portion 122 is divided lengthwise by a partition 108, whichextends upwardly from housing shell 102 of bottom portion 122, to createlower distillation zones 124 and 126. The partition has a height 154.Height 154 of the partition 108 is less than height 155 of the housingshell 102. The partition 108 prevents the liquid bottom product,described below, in the lower distillation zones from mixing. Each ofthe lower distillation zones 124 and 126 have trays 104 and 106,respectively, that are conventional distillation column trays that havebeen shaped to accommodate the wall of the partition 108 and the housingshell 102. Each of the distillation zones 124 and 126 produce adifferent liquid bottom product (bottom product streams 214 and 282)from different feed streams fed thereto (input streams 238 and 278) tothe fractionation column 210 by virtue of the partition 108 separatingthe distillation zones 124 and 126. The partition 108 prevents theliquid bottom products in the lower distillation zones from mixing.

A potential problem with some prior art split-shell arrangements isleakage across the partition, in particular in the column “sump” nearthe bottom of the column where the liquid bottom product inventories arekept separate. Leakage across the partition, which sometimes occurs dueto imperfect welding joints or metal fatigue, could contaminate theliquid bottom products, which could erode or eliminate the benefits ofthe split-shell design. This is particularly important if one of theliquid bottom products is a product for sale instead of an intermediatestream to be recycled for further processing. The presently disclosedsplit-shell fractionation column 210 addresses this potential leakage.

With continued reference to FIG. 1, input stream 278 is introduced intolower distillation zone 124. Input stream 278, in one embodiment,includes a mixture of C₈ aromatic hydrocarbons, C₉ and higher aromatichydrocarbons (hereinafter referred to as C₉+ aromatic hydrocarbons), andtoluene, for example. Input stream 238 is introduced into the lowerdistillation zone 126. Input stream 238, in one embodiment, includes amixture of C₈ aromatic hydrocarbons and toluene, for example. In oneembodiment, input stream 238 includes a relatively high concentration ofpara-xylene. That is, in one embodiment, the para-xylene concentrationof input stream 238 is higher than the para-xylene concentration ofinput stream 278. Of course, these compositions are merely non-limitingexamples and are provided to illustrate the operation of column 210.

Combining the hydrocarbons from input streams 278 and 238 into a singlestream for fractional distillation in a prior art column (i.e., a columnwithout the partition 108) would result in the undesirable dilution ofthe high purity material in input stream 238. In addition, suchcombining would introduce undesirable C₉+ aromatic hydrocarbons intosubsequent processes. The split-shell design allows the two inputstreams to be distilled separately, while still using only a singledistillation column instead of two separate distillation columns,thereby reducing capital expenditures and operational energy costs.

In the operation of the split-shell fractional column 210, the lightercomponents (i.e., those with a lower boiling point, for example some C₈components and lighter components) introduced to the column 210 viainputs 278 and 238 vaporize at the temperature of the lower portion 122.As such, these lighter components travel upwardly in column 210. In oneembodiment, the toluene present (C₇) is extracted as a liquid at a sidecut tray (not shown) and light hydrocarbons such as C₆ and lowerhydrocarbons (hereinafter referred to as C₆− hydrocarbons) are extractedas a vapor at an overhead stream 212/242. In another embodiment, thetoluene is extracted as a vapor in the overhead stream 212/242 andcondensed to form a liquid stream. In one embodiment, the stream 212/242includes high purity toluene. In another embodiment, the stream 212/242includes toluene and light hydrocarbons (C₆−).

The heavier components (i.e., those with a higher boiling point, forexample some C₈ components and heavier components) will remain in liquidform and, therefore, will remain in the lower distillation zone 124, ifintroduced by way of input stream 278, or will remain in lowerdistillation zone 126, if introduced by way of input stream 238. Assuch, the heavier fractions of the input stream 278 and input stream 238will remain segregated in the lower portion 122. In one embodiment, thebottom product stream 282 is a mixture of C₈ aromatic hydrocarbons, C₉+aromatic hydrocarbons, and heavier hydrocarbons. In one embodiment, thebottom product stream 214 is a stream including primarily para-xylene.Again, these non-limiting, exemplary stream components are providedmerely to illustrate the operation of the split-shell 210 column in oneembodiment.

The number of trays 103, 104, and 106 in each of the upper portion 120,lower distillation zone 124, and lower distillation zone 126,respectively, vary with the particular product input streams and desiredoutput streams, as will be appreciated by those skilled in the art. Inone embodiment, the number of trays in lower distillation zone 124 isdifferent than the number of trays in lower distillation zone 126, forexample the number of trays in distillation zone 124 may be greater thanthe number of trays in distillation zone 126, or via versa. In analternative embodiment, the number of trays in lower distillation zone124 is the same as the number of trays in lower distillation zone 126.

The locations of input streams 278 and 238 on split-shell fractionationcolumn 210 are selected to prevent any mixing of the heavy (C₈+)constituents across partition 108. In one embodiment, the partition 108extends 4 trays above the highest of the feed trays (not shown), trays128 and 130 being the highest trays in distillation zones 124, 126,respectively. Feed trays, as used herein, refer to the first traysencountered by input streams 238 and 278 upon entry into the column 210.In one embodiment, the partition extends greater than 4 trays above thehighest of the feed trays. In one embodiment, the partition extends lessthan 4 trays above the highest of the feed trays. As used herein, withreference to fractional distillation columns, the term “above” refers toa location in or on the column such that liquid inserted at the locationwill flow down toward the reference point. Similarly, the term “below”refers to a location in or on the column such that liquid inserted atthe location will flow down away from the reference point.

Greater detail is now provided regarding the partition 108 as shown inFIG. 1. The partition 108 is designed so as to mitigate the risk ofleakage and cross-contamination of the liquid bottom products. Thepartition includes two vertical baffles 141, 142 with a gap 143 betweenthese baffles. Any leakage of liquid bottom products through the baffles141, 142 is collected in a space 143 between the baffles 141, 142, andcan be removed by periodic or continuous draining from the column 210via line 145.

The partition 108 further includes a seal plate 144 at the top of thepartition 108 to create a closed system, effectively a “pressure vessel”within the fractionation column 210. The partition space 143, in oneembodiment, is operated at a lower pressure than the column 210 suchthat any and all leakage through either baffle 141, 142 flows from therelatively higher pressure column 210 into the partition space 143.Nitrogen or another suitable inert vapor is used to maintain thepressure inside the partition space 143, and is introduced via line 146.A drain connection is provided to remove liquid accumulation in thebottom of the partition space 143 via line 145.

The exemplary fractionation column 210, and in particular the “pressurevessel” therewithin, is made by welding two vertical baffle plates 141,142 into the fractionation column 210 with a gap, which in oneembodiment can be sized from about 25 mm to about 50 mm, for examplefrom about 30 to about 40 mm, between the two plates 141, 142 to createa partition 108 with space 143 therein. Each plate 141, 142 is weldedalong the vessel shell so that there is no direct fluid communicationbetween the opposite sides of the baffle plates 141, 142. The top of thebaffle plates 141, 142 are welded to the cover plate 144 thateffectively closes the gap and prevents fluid communication between theinside of the partition and the fractionation column 210. A vapor inletconnection is made to introduce nitrogen or appropriate inert vapor tomaintain an operating pressure within the partition space 143 that islower than the operating pressure of the column. An outlet connectionwith an output port is made at the bottom of the partition space 143 toremove any liquid that may accumulate inside the partition space 143 dueto unintended leakage through the baffles 141, 142, for example due toan imperfect weld connection.

ILLUSTRATIVE EXAMPLE

The following example is merely provided to illustrate one possibleimplementation of a split-shell fractionation column in a broaderaromatic hydrocarbon processing system. As such, the form and content ofthe various material streams are intended to serve only as anon-limiting example for the skilled artisan to better understand theoperation thereof.

FIG. 2 illustrates an embodiment 200 of an aromatic hydrocarbonprocessing system that includes split-shell fractionation column 210,described above. A feed stream 202 enters a xylene fractionation unit204. In one embodiment, the feed stream 202 contains ortho-, meta-, andpara-xylene isomers. In one embodiment, the feed stream 202 containsquantities of ethylbenzene, toluene, C₈ cycloalkanes, alkanes, andhydrocarbons having more than eight carbon atoms per molecule. In oneembodiment, the feed stream 202 is a result of hydrotreating naphtha toremove any sulfur and nitrogen contaminants and the subsequent catalyticreforming where paraffins and naphthenes in the decontaminated naphthaare converted to aromatic hydrocarbons. Most C₇− fractions are removedin a debutanizer and fractional distillation column, respectively.

The feed stream 202, including a C₈+ fraction, enters the xylenefractionation unit 204. In one embodiment, the feed stream 202 includesabout 23 weight percent (wt %) para-xylene. The xylene fractionationunit 204 is a fractional distillation column. The xylene fractionationunit 204 divides the incoming stream into an overhead stream 206including the C₈− aromatic hydrocarbons, including the xylene isomers,ethylbenzene, and toluene, a bottom product stream 208, and one or moreside cut streams (not shown) including C₉+ aromatic hydrocarbons and anyC₇− fractions present in the feed stream 202.

The overhead stream 206 from xylene fractionation unit 204, and a bottomproduct stream 214 from extract column 210, enter adsorptive separationunit 216 at a first feed input 286 and a second feed input 284,respectively. Adsorptive separation unit 216 separates the incomingstreams 206 and 214 into a raffinate stream 218 and an extract stream220. In one embodiment, the heavy desorbent para-diethylbenzene is usedto facilitate the separation of the raffinate stream 218 and extractstream 220. The raffinate stream 218 includes ethylbenzene, meta-xylene,and ortho-xylene diluted with desorbent. The extract stream 220 includespara-xylene diluted with desorbent.

In one embodiment, adsorptive separation unit 216 includes a simulatedmoving bed (SMB) assembly and a rotary valve. The SMB assembly includesa single physical chamber. In one embodiment, the physical chamberincludes 24 beds. In an alternative embodiment, the physical chamberincludes less than 24 beds. In another embodiment, the SMB assemblyincludes two physical chambers. In one embodiment, each physical chamberincludes 12 beds. In an alternative embodiment, each physical chamberincludes more or less than 12 beds. In one embodiment, the physicalchambers have an unequal number of beds. A bed line connects each bed inthe SMB assembly to the rotary valve. The rotary valve controls the flowof material into or out of the SMB assembly in a step- wise manner tocreate a simulated moving bed and to flush the bed lines between flowsof differing materials.

As a mixture of xylene isomers is fed into adsorptive separation unit216, and flows downwardly under the force of gravity, the mixture ofxylene isomers contacts a solid, zeolitic adsorbent within the chamber.The zeolitic adsorbent disposed in adsorptive separation unit 216 has anaffinity for para-xylene. As the mixture of xylene isomers flows overthe solid adsorbent, the para-xylene is selectively adsorbed into theadsorbent while the other isomers continue to travel downward in thechamber in the bulk liquid. In certain embodiments, the selectivity ofthe adsorbent in the adsorptive separation unit 216 for C₇− aromatichydrocarbons and lighter hydrocarbons is very close to that ofpara-xylene. As such, the C₇− aromatic hydrocarbons and lighterhydrocarbons exit the adsorptive separation unit 216 by way of extractstream 220. The extract stream 220 enters the extract column 224.Extract column 224 is a fractional distillation column that separatesthe incoming stream 220 into an overhead para-xylene stream 226including para-xylene, C₇− aromatic hydrocarbons, and lighterhydrocarbons and a bottom product stream 228 including a heavy desorbentfraction, such as para-diethylbenzene (a C₁₀ aromatic hydrocarbon). Thebottom product stream 228 is recycled back to the adsorptive separationunit 216 through combined stream 230.

Light desorbent enters adsorptive separation unit 234 by way of combinedstream 232. Adsorptive separation unit 234 separates an incoming stream254 into a raffinate stream 236 and an extract stream 238. Stream 254 isan isomerized stream from isomerization unit 250 including anequilibrium mixture of xylene isomers. In one embodiment, the lightdesorbent toluene is used to facilitate the separation of the raffinatestream 236 and extract stream 238. The raffinate stream 236 includesethylbenzene, meta-xylene, and ortho-xylene diluted with desorbent. Theextract stream 238 includes para-xylene diluted with desorbent.

In one embodiment, adsorptive separation unit 234 includes an SMBassembly and a rotary valve. In one embodiment, the SMB assemblyincludes a single physical chamber. In one embodiment, the physicalchamber includes 24 beds. In an alternative embodiment, the physicalchamber includes less than 24 beds. In one embodiment, the SMB assemblyincludes two physical chambers. In one embodiment, each physical chamberincludes 12 beds. In an alternative embodiment, each physical chamberincludes more or less than 12 beds. In one embodiment, the physicalchambers have an unequal number of beds. A bed line connects each bed inthe SMB assembly to the rotary valve. The rotary valve controls the flowof material into or out of the SMB assembly in a stepwise manner tocreate a simulated moving bed and to flush the bed lines between flowsof differing materials.

As a mixture of xylene isomers is fed into adsorptive separation unit234, and flows downwardly under the force of gravity, the mixture ofxylene isomers contacts a solid, zeolitic adsorbent within the chamber.The zeolitic adsorbent disposed in adsorptive separation unit 234 has anaffinity for para-xylene. As the mixture of xylene isomers flows overthe solid adsorbent, the para-xylene is selectively adsorbed into theadsorbent while the other isomers continue to travel downward in thechamber in the bulk liquid. The raffinate stream 236 enters a raffinatecolumn 222 at a third location 276. The extract stream 238 and theoutput 278 from aromatic conversion unit 280 are fed into thesplit-shell extract column 210 (thereby become input streams 238, 278 tothe column 210), which was described in greater detail above with regardto FIG. 1, at a first input port 290 and second input port 288,respectively. The split-shell column 210 separates the input streamsinto the previously described overhead stream 212 at third output 296and the previously described bottom product streams 214 and 282 at firstoutput port 294 and second output port 292, respectively. As previouslynoted, the overhead stream 212/242, in one embodiment, includesprimarily toluene. In one embodiment, stream 212/242 includes alsoincludes C₇− aromatic hydrocarbons and lighter hydrocarbon impurities.The bottom product stream 214 includes C₈ aromatic hydrocarbon isomers,including a high concentration of para-xylene (as compared to stream282). The bottom product stream 282 includes C₈ aromatic hydrocarbonisomers. In one embodiment, the bottom product stream 282 has a lowerconcentration of para-xylene than does bottom product stream 214. Thelight desorbent, in one embodiment toluene, is recycled in a lightdesorbent loop 212, 232, 238. In one embodiment, a slipstream 242 isextracted from the overhead stream 212/242. In one embodiment,slipstream 242 prevents the accumulation of additional tolueneintroduced into the desorbent loop from the feed stream 202. In oneembodiment, slipstream 242 prevents the accumulation of lighthydrocarbon impurities in the light desorbent loop. In one embodiment,slipstream 242 includes high purity toluene. In one embodiment,slipstream 242 includes toluene and light hydrocarbon impurities fromthe feed stream 202.

Raffinate column 222 is a fractional distillation column that separatesthe raffinate stream 236 and 218, each including para-xylene depleted C₈aromatic hydrocarbon isomers diluted with light and heavy desorbent,respectively, into a C₈ aromatic hydrocarbon isomer stream 244, a lightdesorbent stream 246, and a heavy desorbent stream 248. The C₈ aromatichydrocarbon isomer stream 244 exits the raffinate column 222 at a secondlocation 274. The light desorbent along with any light impurities havethe lowest boiling point and are, as such, extracted as a net overheadstream 246. The heavy desorbent along with any heavy hydrocarbons (C₉+)have the highest boiling point and are, as such, extracted as a netbottom product stream 248. The ortho-xylene, meta-xylene, andethylbenzene have an intermediate boiling point and are, as such,extracted at a sidecut tray. The heavy desorbent is recycled in a heavydesorbent loop 230, 220/218, 228/248. In one embodiment, the C₈ aromaticisomer stream 244 exits the raffinate column 222 at a location belowthat of raffinate stream 236 and above that of raffinate stream 218. Inone embodiment, the raffinate stream 236 enters raffinate column 222 ata location on the column where the composition within the column 222 issimilar to the composition in stream 236. In one embodiment, theraffinate stream 218 enters raffinate column 222 at a location on thecolumn where the composition within the column 222 is similar to thecomposition in stream 218.

The C₈ aromatic hydrocarbon isomer stream 244 including meta-xylene,ortho-xylene, and ethylbenzene enters an isomerization unit 250.Catalysts in the isomerization unit 250 reestablish an equilibriummixture of the ortho-, meta-, and para-xylene isomers. In oneembodiment, the catalyst is an ethylbenzene dealkylation catalyst, whichconverts ethylbenzene to a benzene co-product. In one embodiment, thecatalyst is an ethylbenzene isomerization catalyst, which converts theethylbenzene into additional xylene isomers. Non-aromatic compounds inthe C₈ aromatic hydrocarbon isomers stream 244 are “cracked” (C—C bondsbroken) to lighter hydrocarbons and removed in stream 252 along with anybenzene co-product created. The isomerization process may also createsmall quantities of C₉ and heavier aromatic hydrocarbons. In oneembodiment, the output stream 254 includes an equilibrium mixture ofxylene isomers. In one embodiment, the output stream 254 includes smallquantities of C₉+ aromatic hydrocarbons. In one embodiment, the outputstream 254 includes unreacted ethylbenzene. In one embodiment, theoutput stream 254 includes about 1.5 wt. % ethylbenzene or less. Theisomerized output stream 254 enters adsorptive separation unit 234.

In certain embodiments, some C₉+ aromatic hydrocarbons may be introducedas a result of the isomerization of ortho-xylene, meta-xylene, andethylbenzene at isomerization unit 250. Any C₁₀+ hydrocarbons willaccumulate in the heavy desorbent loop 230, 220/218,228/248. In certainconfigurations of the raffinate column 222, any C₉ aromatic hydrocarbonswill accumulate in the isomerization loop 254,236,244. In otherconfigurations of the raffinate column 222, any C₉ aromatic hydrocarbonswill accumulate in the heavy desorbent loop 230, 220/218, 228/248. Inyet other configurations of the raffinate column 222, any C₉ aromatichydrocarbons will accumulate in both the isomerization loop and theheavy desorbent loop. In different embodiments, one or more drag streamsare used to prevent the accumulation of C₉+ aromatic hydrocarbons in theprocess. In one embodiment, if accumulation occurs in the heavydesorbent loop, a drag stream 264 is withdrawn from the desorbent loopby way of stream 230. Stream 230 includes primarily heavy desorbentalong with the C₉ aromatic and heavier hydrocarbon impurities. The dragstream 264 is fed into a fractional distillation column 266, whichseparates the drag stream 264 into an overhead stream 268 and a bottomproduct stream 270. The bottom product stream 270 includes high puritypara-diethylbenzene, which is returned to the desorbent loop by way ofstream 230. In one embodiment, the amount of material withdrawn in dragstream 264 is about 1 to about 20 volume percent of stream 230. Inanother embodiment, if accumulation occurs in the isomerization loop(i.e., 254, 236, 244), a drag stream 262 is withdrawn from theisomerization loop by way of raffinate stream 244. Stream 262 includes amixture of ortho-xylene, meta-xylene, ethylbenzene along with the C₉aromatic and heavier hydrocarbon impurities. In one embodiment, theamount of material in the drag stream 262 is about 1 to about 20 volumepercent of the raffinate stream 244. In yet another embodiment, if theaccumulation occurs in both the isomerization loop and the heavydesorbent loop, drag streams 262 and 264 are both used. In otherembodiments, no drag streams are used. In other embodiments, theimpurities are extracted by another process known in the art capable ofseparating C₉ aromatic hydrocarbons and heavier hydrocarbons frompara-diethylbenzene.

In one embodiment, the aromatic conversion unit 280 converts theincoming stream 262, including a mixture of toluene and C₉+ aromatichydrocarbons, into an output stream 278 including an equilibrium mixtureof xylene isomers, ethylbenzene, and toluene. The aromatic conversionunit 280 facilitates catalytic disproportionation reactions, whichconvert toluene into a mixture of benzene and xylene isomers. Thearomatic conversion unit 280 also facilitates catalytic transalkylationreactions, which convert a blend of toluene and C₉ aromatic isomers toxylene isomers through the migration of methyl groups betweenmethyl-substituted aromatic hydrocarbons. Benzene produced in thearomatic conversion assembly is extracted in an additional stream (notshown). The output stream 278 is fed into the split-shell extract column210 of the present disclosure (described in greater detail above withregard to FIG. 1), which separates the output stream 278 into anoverhead stream including toluene and a bottom product stream 282including C₈+ aromatic hydrocarbons. The bottom product stream 282 isfed back into the xylene fractionation column 204 to separate the C₈aromatic hydrocarbons into stream 206 and the C₉+ aromatic hydrocarbonsinto stream 208. The overhead toluene stream from the split extractcolumn 210 is split into streams 212 and 242. Stream 242 is recycledback into the aromatic conversion unit 280 for transalkylation. Stream212 is part of the light desorbent loop 212, 232, 238.

The finishing column 272 separates the overhead stream 226 from extractcolumn 224 into an overhead stream 274 including C₇− aromatichydrocarbons and lighter hydrocarbons, and a bottom product stream 276including high purity para-xylene. In certain embodiments,para-ethyltoluene, structurally similar to para-xylene, may beintroduced into the process by the isomerization unit 250. In someembodiments, the para-ethyltoluene is separated from the para-xylene inthe adsorptive separation unit 216, in the extract column 224, or in thefinishing column 272. In some embodiments, the para-ethyltoluene isremoved from the para-xylene product using techniques known in the art.In one embodiment, the bottom para-xylene stream 276 includes about 95.0wt. % para-xylene. In one embodiment, the bottom para-xylene stream 276includes about 99.2 wt. % para-xylene. In one embodiment, the bottompara-xylene stream 276 includes about 99.7 wt. % para-xylene. In oneembodiment, the bottom para-xylene stream 276 includes about 99.9 wt. %para-xylene. In one embodiment, the bottom para-xylene stream 276includes greater than about 99.9 wt. % para-xylene.

Accordingly, an improved split-shell fractionation column has beendescribed. The improved column beneficially mitigates the risk ofleakage and cross-contamination of liquid bottom products at the bottomof the split-shell fractionation column. Furthermore, the improvedcolumn desirably reduces capital and energy costs by combining twofractionation columns into a single column that has a common overheaddistillate product but dissimilar liquid bottom products.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the claimed subjectmatter in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the described embodiment or embodiments. It should beunderstood that various changes can be made in the processes withoutdeparting from the scope defined by the claims, which includes knownequivalents and foreseeable equivalents at the time of this disclosure.

What is claimed is:
 1. A process for separating aromatic hydrocarbons,comprising the steps of: introducing a first stream comprising aplurality of aromatic hydrocarbons into a first distillation zone of asplit-shell fractionation column; introducing a second stream comprisinga plurality of aromatic hydrocarbons into a second distillation zone ofthe split-shell fractionation column, the first and second distillationzones being defined by a partition within the split-shell fractionationcolumn, the partition having a gap region therein; separating the firststream into a first overhead product and a first bottom product, whereinthe first bottom product comprises a first liquid that collects at abottom portion of the first distillation zone; separating the secondstream into a second overhead product and a second bottom product,wherein the second bottom product comprises a second liquid thatcollects at a bottom portion of the second distillation zone; drainingthe first liquid, the second liquid, or a combination thereof in the gapregion through a first outlet port.
 2. The method of claim 1, furthercomprising introducing nitrogen gas into the gap region.
 3. The methodof claim 2, further comprising maintaining the gap region at a gaspressure that is lower than a pressure of either the first or seconddistillation zones.
 4. The method of claim 1, further comprisingdraining the first bottom product from the bottom portion of the firstdistillation zone through a second outlet port.
 5. The method of claim1, further comprising draining the second bottom product from the bottomportion of the second distillation zone through a third outlet port. 6.The method of claim 1, further comprising removing the first and secondoverhead products through a fourth outlet port located at a top portionof the split-shell fractionation column.